Piezoelectric elements are used as driving components in positioning mechanical devices. These “piezoelectric motors” (sometimes called “piezoelectric actuators”) have some unique advantages over other types of traditional motors which make them a preferred choice for use in specific applications. The typical use of a piezoelectric element as a motion device is by using its characteristic of expanding in one direction when placed in an electric field. By stacking multiple thin piezoelectric elements between a series of electrical electrodes it is possible to increase the relative expansion of a piezoelectric actuator but the motion still remains extremely small relative to the overall size of the device. For applications that require moves with travel ranges less than a few hundred microns and resolution in the nanometer range, the piezoelectric actuators are the usual favorite. These implementations may or may not use a mechanical amplifier (typically using the “lever arm” principle) and are directly coupled to the load. Piezoelectric-based motion devices using this type of driving mechanism typically use flexure-based mechanisms and are capable of fast speeds and resolutions less than a nanometer.
To achieve a longer travel range, one type of piezoelectric motor that has been developed is a friction-based configuration where one or more friction “legs” are in contact with a friction “track”. By applying a constant high pressure between the legs and the track, the friction legs can “stick” to the friction track. A piezoelectric element is attached to either the friction “legs” or “track”. When a slow varying voltage is applied to the piezoelectric element, the respective friction element will move. Due to the high friction forces, the two friction elements will “stick” and move together. If the electrical signal driving the piezoelectric elements changes very rapidly, the friction elements will “slip” relative to each other due to the inertia of the masses associated with the components involved. While the actual motion happens during the slow movement phase of the piezoelectric element, the slippage changes the relative position between the friction legs and the friction track effectively allowing the piezoelectric element to add a new displacement to the previous one. Large travel distances can be achieved by repeating this cycle many times and effectively “stitching” together the small piezoelectric moves. The travel length of this type of piezoelectric motor is typically limited by only the length of the friction track. The track could be linear or circular, allowing the motor to generate a linear or rotary motion. These types of motors are typically called piezoelectric “stick-slip” motors.
The most popular current implementations of stick-slip motors can be grouped in two categories, based on the configuration of the friction legs relative to the friction track. The first type arranges the friction legs around the friction track, effectively forming a clamp on a track shaped as a rod. These motors have the advantage of a concentric configuration, allowing high preload forces to be applied without stressing the motion devices to which they are mounted. The main disadvantages are that the preload force cannot be adjusted and the travel could be limited by the length of the rod. The piezoelectric element can be attached to either the clamp or the rod, with advantages and disadvantages to each configuration.
In the second type of stick-slip motor, the piezoelectric element is typically attached to one or more friction legs which are in contact with the friction track only on one side. This allows the active part of the motor (piezoelectric element and friction leg) to be built as a separate unit that could be tangentially attached to any track, linear or rotary. These types of motors are more versatile but have the disadvantage of generating lower forces and adding stresses to the bearings of the motion devices to which they are attached.